The semiconductor nanocrystals (NCs) are the semiconductor nanocrystals of 1-100 nm. Because the dimension of the semiconductor nanocrystals is smaller than the bulk exciton Bohr radius of other materials, and shows the strong quantum confinement effect to develop the quasi continuous energy band into the disjunct level structure similar as the molecules', and appears the new material property, which is called the quantum dots (QDs).
Due to the external energy excitation (such as photoluminescence, electroluminescence and cathodoluminescence), the electron jumps from the ground state to the excited state, and the electron and hole in the excited state may form the exciton; the electron and hole has the recombination occurs, and ultimately has the relaxation to the ground state; the excess energy is released through the recombination and the relaxation, and the radiative recombination may occurs to emit photons.
The Quantum Dots Light Emitting Diodes (QD-LEDs) possesses important value of business applications and is drawing the strong interest in research of the people in the latest decade. As a matter of fact, the QD-LEDs has many advantages than the Organic Light Emitting Diodes (OLEDs): (1) the line width of the quantum dots luminescence is 20-30 nm, and relative to the luminescence of organic light emitting >50 nm, the FWHM (full width at half maximum) is narrower, which acts the key function of realizing the color purity of the image; (2) the inorganic material has better thermal stability than the organic material, and as the element is in condition of high brightness or high current density, the Joule heat is the main reason of the element degradation, and with the excellent thermal stability, the inorganic material based element shows longer usage life time; (3) because the life times of the organic materials of red, green, blue, three primary colors are different, the color of the OLED display will change with times but once one material is employed to compose the quantum dots of various sizes, and due to the quantum confinement effect, the luminescence of the three primary colors can be achieved, and the one material can appear the similar degradation life time; (4) the QD-LEDs can realize the infrared emission but the luminous wavelength of the organic material is generally smaller than 1 micrometer; (5) there is no spin statistical constraint for the quantum dots, and the external quantum efficiency (EQE) may reach up to 100%, and the EQE of the QD-LED can be represented ηExt=ηr*ηINT*η*ηOUT, wherein ηr is the probability that the electron and hole in the excited state form the exciton, and ηINT is the internal quantum efficiency, i.e. the luminescence quantum yield (PLQY), and η is the probability of the radiative transition, and ηOUT is the external coupling efficiency. The restriction of the organic fluorescent dyes ηr is 25%, wherein the formation ratio of the single state and the triple state is 1:3, and only the recombination of the single state exciton can result in luminescence. However, due to the spin-orbit coupling, the ηr of the organic phosphorescent material is larger than 25%. Significantly, the organic phosphorescent material leads to the degradation of the matrix material, and ηOUT of the planar light emitting device is about 20%, and the external coupling efficiency can be raised with micro cavity structure. For the QD-LEDs, ηINT can reach up to 100%, and as energy levels of the electron and hole are proper, ηr also can reach up to 100%.
The ink-jet printing is a kind of printing technology of no contact, no pressure, no printing plate. It has the common feature of the digital printing and can realize the printings of kinds of inks. The basic principle is the picture and text information stored in the computer is inputted to the flat printer. Then, with the special device under control of the computer, the nozzle jets ink drops to the surface of the object for printing. After the ink drops spread and is dried, the thin film is formed in the desired position.
Because the ink jet printing technology possesses advantages of high material usage ratio, no need for the mask and suitability of large size panel production, the quantum dots material can be formulated to be ink, and the ink jet printing technology can be employed to manufacture the thin film elements. For making the ink suitable for the ink jet printing technology, the viscosity and the surface tension of the ink is adjusted. The common practice is to add the surfactant. However, after the ink is dried, and once the surfactant is residual in the thin film, the bad influence will exist to the performance of the thin film. The surfactant is equivalent to the impurity in the thin film, which will have influence to the conductivity and light emitting performance of the thin film.